NOx monitor using differential mobility spectrometry

ABSTRACT

System for detection and analysis of gas samples in fieldable real-time Differential Mobility Spectrometry (DMS) chemical sensor system which uses non-radioactive ion source for monitoring and detecting NOx emissions; provides reliable methods for detecting and monitoring of anthropogenic sources of NOx; also detection of NO in exhaled breath for patient health diagnosis.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 60/418,235, filed Oct. 12, 2002, entitled FAIMS METHOD AND APPARATUSFOR NOX DETECTION AND ANALYSIS, by Raanan A. Miller, Erkinjon G.Nazarov, and Muning Zhong. The entire teachings of the above applicationare incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to spectrometry, and more particularly, tospectrometer devices providing chemical analysis by aspects of ionmobility in an electric field.

Chemical detection systems are used in a wide array of applications.These devices may take samples directly from the environment, or mayincorporate a front end device to separate compounds in a sample beforedetection. There is particular interest in providing a chemicaldetection system capable of accurate compound detection andidentification, and which may be deployed in various venues, whether inthe lab, in the workplace or in the field.

Mass spectrometers are well known as the gold standard oflaboratory-based systems operate at low pressures, resulting in complexsystems, and the spectra output can be difficult to interpret, oftenrequiring a highly trained operator.

At times a gas chromatograph (GC) is used as a front-end to an MS, withgood results. But the GC-MS is not well-suited for small, low cost,fieldable instruments for real-time chemical detection. Neverthelessthere is a continuing need for fieldable instruments generation ofreal-time detection data.

Detection of species of NOx is a good example of this need. It is wellknown that reactive nitrogen oxide species NOx such as NO, NO₂, and NO₃play a major role in atmospheric chemistry. These species are importantin the ozone and nitrogen cycles, which produce detrimentalphotochemical smog and acid rain. Tougher environmental regulations toreduce these levels in the atmosphere require higher performancedetectors and monitors applied to anthropogenic sources of NOx (e.g.exhaust from internal-combustion engines, steel mill processing, powerplant emissions, etc.) with higher sensitivity and faster responsetimes. Enhancement of fuel economy of internal-combustion engines isanother driver for the development of sensors which are able toprecisely, and rapidly, monitor NOx levels in exhaust emissions.

The medical value of detection of NO in exhaled breath has also beenrecognized for clinical diagnosis. For example, it has been reportedthat such clinical analysis can provide a noninvasive window into theactivities of disease, such as asthma, chronic obstructive pulmonarydisorder, and cystic fibrosis, in the lower airways. There is thereforea desire for improved, portable and simple apparatus and method forevaluation of NO in exhaled breath for medical purposes.

There are a number of reliable measurement techniques which have beendeveloped for monitoring nitrogen oxide species: These include ion-baseddetection, chemiluminesence, electrochemical, acoustic gas sensors, andZrO₂ solid electrolyte sensors, laser systems, and the like. Thesetechniques, however, generally require sophisticated optical equipment,or suffer from significant drawbacks such as slow response times, or thedetection of only certain NOx species, making them problematic forroutine measurements.

Ion Mobility Spectrometry (IMS) has been explored recently as anapproach to realizing a more sensitive, selective and robust device forNOx monitoring. In dry (humidity ˜10 ppm) operating conditions, the IMSshows high sensitivity (10's of ppms) and fast response times (10's ofms). However, a serious disadvantage is that its response is highlyaffected by the presence of moisture. For example, in one demonstration,a level of 3% humidity completely suppressed IMS response to a sample at483 ppm NO₂.

Time-of-flight Ion Mobility Spectrometers (TOF-IMS) are considered to befunctional chemical detectors. High-speed response and low memoryeffects have been attained, and the gas phase ion chemistry inside theTOF-IMS can be highly reproducible. Widespread use, however, stillremains a problem for TOF-IMS. Among other things, TOF-IMS flow channels(also referred to as drift tubes) are still comparatively large andexpensive and suffer from losses in detection limits when made small.

The differential ion mobility spectrometer ((DMS), also known as a highfield asymmetric waveform ion mobility spectrometer (FAIMS)), is analternative to the IMS. In a DMS device, a gas sample that contains achemical compound is subjected to an ionization source. Ions from theionized gas sample are drawn into an ion filter region, where the ionsflow in a compensated high asymmetric RF field generated between filterelectrodes, the field being transverse to the ion flow. The field iscompensated to allow selected ion species to pass through the filter,based on aspects of their mobility in the field. These ion species arepassed downstream to an ion detector. Detections are correlated withfield conditions and compensation and species identification is made byreference to known species behavior in the extant compensated DMS field.

The asymmetric field alternates between a high and low field strengthcondition that causes the ions to move in response to the fieldaccording to their mobility characteristics. Typically the mobility inthe high field differs from that of the low field. That mobilitydifference produces a net transverse displacement of the ions as theytravel in the gas flow through the filter. This transverse travel of theions continues until they drive into one of the filter electrodes andare neutralized. However, the field is also compensated such that aparticular ion species will remain toward the middle of the flow in theflow path and will pass through the filter without neutralization. Theamount of change in mobility in response to changes in the asymmetricfield is compound-dependent. This permits separation of ions from eachother according to their species based on the applied compensation(usually a dc bias applied to the filter electrodes).

In the past, Mine Safety Appliances Co. (MSA) made an attempt at afunctional cylindrical FAIMS device with coaxial electrodes, such asdisclosed in U.S. Pat. No. 5,420,424. (This FAIMS technology is referredto by MSA as Field Ion Spectrometry (FIS).) The device has been found tobe complex, with many parts, and somewhat limited in utility.

It is a therefore an object of the present invention to provide afunctional, small, spectrometer that overcomes the limitations of theprior art.

It is another object of the present invention to provide a chemicalsensor with fast response times for real-time process control,especially for detection and identification of NOx related species inreal-time.

It is another object of the present invention to provide a chemicalsensor for detection and identification of NOx related species inreal-time with minimized effect of moisture upon detection results.

It is yet another object of the present invention to provide low costand compact, reliable instrumentation that is useful for laboratory andfield conditions and is capable of making in situ measurements ofchemicals present in complex mixtures at various venues.

SUMMARY OF THE INVENTION

The present invention achieves non-radioactive detection andidentification of trace amounts of NOx species in a gas sample. Invarious embodiment of the invention, method and apparatus (i.e.,systems) are provided for non-radioactive detection and quantificationof NOx in a gas sample. Systems of the invention are compact, with fastresponse times (msec scale), high sensitivity and specificity, and lowmanufacturing cost. Devices of the invention are capable of beingmass-produced with broad applicability.

Systems of the invention can detect positive NO spectra and negative NO₂(and even NO₃) spectra. This detection can be performed simultaneouslyin a single scheme. Also, total NOx can be detected upon addition of O₂gas to the sample to be scanned.

In practice of the invention of preferred embodiments of the invention,it is possible to detect NOx constituents in a single analytical scheme.A simple, practice of the invention includes Differential MobilitySpectrometry (DMS) systems which operate rapidly and can provideanalytical information in real-time. These systems preferably featurenon-radioactive ionization, which may be by means of a UV lamp, bycorona discharge, by plasma, or by other photon sources. Use of anon-radioactive ionization source reduces risk to users and avoidsregulatory issues as well as reducing the ionization energy to a levelthat avoids fragmenting or otherwise damaging the sample analytes to bedetected.

Illustrative applications of the invention include detection of NOx incombustion exhaust gas for environmental monitoring purposes or as partof a controller for improving the combustion process. Such innovationaddresses the analytical challenge posed by increasingly strictenvironmental regulations applied to anthropogenic sources of NOx (e.g.,exhaust from internal-combustion engines, steel mill processing, powerplant emissions, etc.).

Still another illustrative application of the invention includes asystem for non-radioactive detection of NO in exhaled breath. Suchdetection system enables non-invasive and real-time patient healthevaluations, and can be provided in a compact and low-cost package.

It has also been found that DMS practices of the invention minimize theaffect of moisture on analyte detection. In one practice of theinvention, moisture in a NOx sample does not affect DMS response up toabout 1000 ppm and above 1000 ppm the compensation voltage increaseswith increasing humidity. Quite advantageously, this results in anenhanced resolving ability of the DMS in with varying levels ofmoisture.

In an illustrative DMS practice of the present invention, a gas sampleis ionized. Ions from the ionized gas sample are drawn into a DMS ionfilter and are subjected to differential ion mobility filtering todetermine presence and amount of NOx.

This DMS filtering accentuates differences in ion mobility of theionized sample in a high-low alternating asymmetric RF field. The filterfield is compensated such that selected ion species are allowed throughthe field and are passed to an ion detector. Passing of an ion speciesis based on high field ion mobility characteristics of the species inthe changing filter field conditions. All other species are neutralizedwithin the filter.

Ion species identification itself follows upon downstream detection ofthe passed ion species and comparison of detection results against knowndetection behavior for the particular field conditions. In a preferredembodiment of the invention, positive and negative ion species aredetected simultaneously.

Embodiments of the invention employ compact DMS systems made accordingto the principles of 1) U.S. Pat. No. 6,495,823, entitled MICROMACHINEDFIELD ASYMMETRIC ION MOBILITY FILTER AND DETECTION SYSTEM, by Raanan A.Miller and Erkinjon G. Nazarov, incorporated herein by reference, 2)U.S. patent application Ser. No. 10/187464, filed Jun. 28, 2002,internal Attorney Docket M070, entitled SYSTEM FOR COLLECTION OF DATAAND IDENTIFICATION OF UNKNOWN ION SPECIES IN AN ELECTRIC FIELD, byLawrence A. Kaufman, Raanan A. Miller, Erkinjon G. Nazarov, EvgenyKrylov, Gary Eiceman, incorporated herein by reference, and/or 3) U.S.patent application Ser. No. 10/462206, entitled SYSTEM FOR COLLECTION OFDATA AND IDENTIFICATION OF UNKNOWN ION SPECIES IN AN ELECTRIC FIELD, byLawrence A. Kaufman, Raanan A. Miller, Erkinjon G. Nazarov, EvgenyKrylov, Gary Eiceman, incorporated herein by reference.

These and other aspects of the present invention are set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed and other objects, features and advantages of theinvention will be apparent from the following description ofillustrative and preferred embodiments of the invention, and asillustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention, wherein:

FIG. 1A is a diagram of a fast DMS combustion control system in practiceof the invention.

FIG. 1B is a diagram of a fast DMS system for breath analysis.

FIG. 2(a-d) shows four related DMS scans of an NOx sample: (a) showingpresence of NO and NO2, in N2 carrier gas; (b) same, with 1% O2 added tocarrier; (c) same, taken 4 seconds after O2 removed from gas flow of(b); and (d) same, 18 seconds after shut off of O2 of (b).

FIG. 3 shows spectra for (a) NO and (b) NO₂ in clean nitrogen transportgas, with RF voltage at 1200V; the upper scan shows the spectrum forpositive ions and the lower scan shows the negative response in each.

FIG. 4 shows concentration dependence of DMS for (a) nitric oxide and(b) nitrogen dioxide, comparing peak area (squares) to peak height(diamonds).

FIGS. 5 a, 5 b, 5 c and 5 d show DMS spectra for negative (lower traces)and positive (higher traces) ion species for NOx samples, with transportgas at a mixture of: 23.6 ppm of SO₂; 121.2 ppm of H₂; 398 ppm of CO,8.1% of O₂; 10% of CO₂; and N₂ as the balance gas.

FIG. 6 shows the effect of changing NOx concentration on DMS Spectra.

FIG. 7 shows the effect of humidity on negative mode NOx peak positionand peak intensity for transport gases of clean nitrogen and the complexmixture discussed in section e.

FIG. 8 shows the response of DMS to a transport gas composed of propene,O2, N2 and water.

FIG. 9 shows the response of DMS when NO₂ and NO samples are added tothe DMS transport gas.

FIG. 10 is a schematic of an illustrative DMS/MS interface inapplication of the invention.

FIG. 11 shows mass-spectra for positive NOx ions: (a) 20 ppm of NO indry N₂ transport gas, (b) 20 ppm of NO in dry N₂+O₂ (10%) transport gas,similar spectra obtained with NO₂ in dry N₂, and (c) 20 ppm of NO inhumidified N₂ transport gas.

FIG. 12 shows mass-spectra for negative NOx ions: (a) 20 ppm of NO₂ indry N₂; and (b) 20 ppm of NO in humidified N₂, and similar spectra whenNO₂ sampling.

FIG. 13 is a table of ionization energies and proton and electronaffinities.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

We are able to detect NOx, NO₂, N2O4, HNO3, among other constituents, ina gas sample of NOx, and we are able to distinguished between suchconstituents in a single analytical framework. Systems of the inventionobtain actual measurement of NOx activity in a changing environment inreal-time. Applications include combustion control, whether forautomotive engines or commercial furnaces or locomotives, or marineengines or the like, or medical tools, for example.

Turning to FIG. 1A, in a preferred illustrative embodiment of theinvention, a DMS system 10 is coupled to a combustion 11 (such as in afactory or generator or an automotive engine or the like) for real-timecontrol of combustion conditions resulting in cleaner combustion andimproved fuel economy. Fuel and air are drawn into the combustion andare mixed under control of a combustion controller and are combusted ina combustion chamber 12. Exhaust from the chamber includes manycompounds. Of high interest is NOx and its constituents. For example,inefficient burning can result in emission of high levels of HNO3, whichcan result in acid rain. Detection of NO and NO2 is also important incontrolling combustion.

In FIG. 1A, a sample of exhaust S from the combustion chamber 12 isdrawn into an ionization chamber 14. Selection of an ionization sourceis important. We have found that a source that provides soft ionizationconditions is best for NOx detection of combustion components. A higherenergy source can result in ionization of the nitrogen in the backgroundair and therefore can corrupt detection and analysis of NOx constituentsof the combustion effluent. A non-radioactive source is thereforepreferred. A UV source 16 provides adequate energy to ionize the targetNOx without ionization of the nitrous background. A UV lamp is aconventional soft ionization source and would be adequate and evenpreferred in practices of the present invention.

In FIG. 1A, the sample is ionized in chamber 14 and the ionized sampleS⁺ is flowed into the DMS filter 18 wherein it is filtered according toknown DMS principles, as earlier described. Species detection data fromdetector 19 is processed to enable identification of the NOxconstituents. This process is managed under control of a DMS Systemcontroller 20. Based on such identification, a data signal is issued bythe system controller 20 to the combustion controller 22 for improvementof fuel combustion. Such detection, identification and combustionadjustments are enabled by reference to a store of known systembehavior, which may be incorporated within controller 20.

Because we can distinguish the effects of humidity on analyticalresults, we can provide real-world, real-time and accurate detection ofNOx and its constituents, unlike IMS systems. Therefore, the combustioncontroller of the invention performs combustion control functions whichcan be used to improve combustion efficiency in real-time and inreal-life environments, in situ, as will be fully appreciated by aperson skilled in the art.

Other embodiments of the invention enable portable medical equipment fordetection of NO in patient breath as a non-invasive diagnostic, such asfor of lower airway diagnosis. Turning to FIG. 1B, a patent breathoutput is captured as sample S and is introduced into a DMS system 10 bya exhale capture device 13 for real-time detection of NO in the patent'sbreath. As in FIG. 1A, sample S is drawn into an ionization chamber 14and ionization, filtering, detection and identification proceedaccordingly. In an illustrative application, daily measurements of NO inthe breath can be used to regulate medication, such as glucocorticoids,for such diseases as asthma.

An Illustration of the Invention:

An illustrative operation of the invention is discussed with referenceto FIG. 2, in which four related DMS scans of a NOx sample are shown(Frames a-d). FIG. 2(a) shows presence of NO and NO2 in N2 carrier gas(flow rate of 6 ppm) and is a plot of intensity versus compensationvoltage in the DMS filter with detection of NO at intensity of about 3.5at about −32 v compensation and with trace NO2 at about −20 v.

In FIG. 2(b) the conditions of FIG. 2(a) were repeated but with 1% O2added to the gas flow. Reaction of the NOx and O2 results in formationof NO2, as indicated by several intensity peaks (as shown by the arrows)at several compensation levels. It will be observed that the 1% O2resulted in essentially total conversion of NO to NO2 in this situation.

In FIG. 2(c) the conditions of FIG. 2(a) were repeated but and a readingwas taken 4 seconds after the O2 was removed from the gas flow. Owing tosome latency in the system, some amount of O2 remains in the flow pathas indicated by the moderate presence of N02 detected at about −21compensation, and the return of NO as seen at −33 compensation.

In FIG. 2(d), the conditions of FIG. 2(a) were repeated but with 1% O2added to the gas flow and the detection was made 18 seconds after shutoff of the O2. FIG. 2(d) shows that the O2 has cleared the flow path 18seconds after it was removed from the flow. No attempt was made toaccelerate this clearing, as might be done by heating or other purgingprocesses. Nonetheless, this shows that the system has purged and nearlyreturned to the condition of FIG. 2(a). Furthermore, under realisticoperating conditions, at elevated temperatures, the sample would bepurged all the more readily.

Each of these frames shows both positive and negative mode detectionsfor the NOx constituents of the sample in practice of the invention.Positive and negative mode detections are made in practice of apreferred compact DMS embodiments of the invention as taught in Ser. No.10/187464, as incorporated herein.

It will thus be appreciated that in practice of the invention, acontrolled gas flow can be used for detection of NOx and itsconstituents, and that such is achieved with high resolution andspecificity. This process is both repeatable and predictable.

EXAMPLES

Ion energy considerations for photo-ionization and ion formation of NOxspecies in air at atmospheric pressure in practice of the invention areprovided below.

First, an understanding of the ion energetics and ion chemistry isprovided, as this is part of the understanding of operation of thepreferred DMS method and apparatus of the invention and is a foundationfor interpreting the resulting spectra.

As an illustration, FIG. 13 shows the typical chemical composition of anengine exhaust. Select ion energy properties of these components are:ionization energy (EI), electron (EA) and proton affinity (PA), andpotential for ion formation via a UV ionization source with photonenergy of 10.67 eV.

Analysis of the data from FIG. 13 shows that only NO, NO₂, and propenespecies have an ionization energy lower than the photon energy (10.6 eV)provided by the UV ionization lamp used in this work. Consequently onlythese three components of the exhaust gas will directly form ions, inthis case positive ions. The other components cannot directly form ions.However, there are other pathways for ion formation at atmosphericpressure due to proton and electron charge exchange reactions. Forexample in the positive mode, due to proton transfer reactions, weexpect formation of ion species with the highest proton affinity. Theseare propene and protonated water cluster ions. In the negative mode, ionformation will occur as follows: free electrons which are formed due tophoto-ionization of NOx and propene molecules, at atmospheric pressureconditions, will be captured by components which have positive electronaffinity. In our experimental conditions, it may be oxygen, CO, andnitrogen dioxide molecules. NO₂ molecules have significantly higherproton affinity compared with the other components. Given sufficienttime all free electrons will be transferred to the nitrogen dioxidemolecules. Therefore, based only on ion energetic arguments it isexpected that with UV ionization, and monitoring of the DMS negativespectra, it should be possible to detect and monitor different levels ofNO₂ in an exhaust stream.

In an illustrative practice of the invention with UV ionization, aKrypton filled UV lamp (PID Lamp λ=123.9 nm and 116.9 nm with energies10.0 eV and 10.6 eV) was used. Standard gases includes NO 1500 ppmbalanced with nitrogen, NO 1500 ppm, NO2 1500 ppm. Drift Gas was grade 5nitrogen (99.9995%). The drift gas (nitrogen) was flowed at 1500 cc/min,with a range of DMS compensation voltage (CV) between −45V and +12V,with scan time at 1 sec. RF voltage was at 1200V,. Scans to average=1,Steps to average=1, Number of steps/scan=250. Mass flow controllers(MFC) were used at 2000 cc/min, 100 cc/min, 50 cc/min and 10 cc/min fornitrogen and 50 cc/min for oxygen. Performance was at room temperature.Drift gas and NOx standard gases were introduced and mixed via MFCs atdesignated levels. Oxygen at 10% was introduced with a flow controllerfor NO/NO2 dynamic determination.

A preliminary set of control experiments was conducted prior tointroduction of real samples. First, the DMS system was run with atransport gas of pure nitrogen to verify that the background spectrumwas clean. As expected, no peaks were observed in both positive andnegative modes. When small amounts of NOx samples were mixed with thetransport gas, peaks appeared in the DMS spectra which behavedconsistently with ion energetic considerations of the analytes. NOsamples produced dominant positive ion peaks and NO₂ samples exhibitedmostly negative ion peaks.

NO and NO₂ Spectra in Positive and Negative Modes

FIG. 3 shows individual spectra for NO (FIG. 3 a) and NO₂ (FIG. 3 b)samples. As expected, the NO sample spectra shows a single positive peakat Vc=−23V. The low intensity negative peak around Vc=0V is believed tobe an impurity.

The NO₂ sample spectra showed a major single peak in the negative modeat Vc˜−10V. Two peaks with relatively small intensities are alsoapparent in the positive polarity at Vc=−23V (same location as for theNO sample), and at Vc=−17V. These positive peaks are believed related toNO⁺ and NO₂ ⁺ and may be formed due to photolysis of the NO₂ moleculesinto NO, O, and a combination of these molecular species (e.g. NO₃), asshown in equation (1):NO₂+hv→NO+ONO₂+O+M→NO₃+MNO₂+e⁻→NO₂ ⁻  (1)NO₂ ⁻+NO₃→NO₂+NO₃ ⁻NO₂ ⁻+NO₂→NO+NO₃ ⁻

Because molecules of NO₂ and NO₃ have very high electron affinities (2.3and 3.9 eV respectively), these species will capture electrons from theother compounds (including unidentified impurities) due to chargeexchange processes, and form negative ion species. This may explain thepresence of a stable single peak in the negative spectra. In this modelthe intensity of the negative peak should be proportional to theconcentration of NOx in the gas mixture.

It is well known that mixtures of NO and NO₂ exist in dynamicequilibrium with their relative concentrations dependant on experimentalconditions. FIG. 3 illustrates that the DMS has the ability to directlymeasure the amounts of both NO and NO₂ species actually existing in themixture. The positive ion spectra, provides information on the NOspecies while the negative ion spectra provides information on theamount of NO₂ (or NO₃) species present. This was confirmed by avalidating experiment performed by oxidizing NO. 10% of oxygen was addedto the standard nitrogen transport gas and blended with the standard NOsample. This resulted in a decreased intensity of the positive NO ionpeak at Vc=−23V and simultaneously an increase in the intensity of thepositive ion peak at Vc=−17V. Meanwhile in the negative polarity, a newpeak at Vc=−10V, related to NO₂ appeared. After the oxygen flow wasturned off, the peaks transformed back to the characteristic NO spectra.

Concentration Dependences for NO and NO₂ Species

FIG. 4(a & b) shows the DMS intensity response to differentconcentrations of NO (positive polarity ions) and NO₂ (negative polarityions) samples. Plots for peak height (blue) and for peak area (pink) arepresented. One can see that for NO these plots have a linear range(Y=4.6x-6.1 e.g. for area) up to 6 ppm. The NO₂ plots are nonlinear butcan be fitted very well by quadratic approximations(Y=0.0055x²+1.65x-1.9 for area plot). The resulting reactions forpositive and negative ion formation have different reaction orders.Positive ion formation has a first reaction order while the negative ionformation has a second reaction order. This means that the rate ofnegative ion formation is increased with increasing concentration ofNO₂. One of the possible explanations is that in the negative mode NOand NO₂ are oxidized to NO₃ producing the negative spectral peak. Theoxidation process may be enhanced due to the UV lamp radiation,according to reactions (1). Another mechanism which is specific fornitrogen dioxide is due to collision of neutral molecules of NO₂ withoxygen atoms resulting in their transformation from one molecule toother.NO₂(g)+NO₂(g)=NO₃(g)+NO(g)  (2)

With increasing concentration of NO₂ the rate of this reaction increasesdue to increasing collision frequency. In this case, the amount of freeelectrons is increased due to ionization of NO resulting in positiveions (NO has the lowest ionization energy) and a second product NO₃which has the highest electron affinity (3.9 eV). This results in acondition favoring increased efficiency of negative ion formation. Theestimated limit of detection for both chemicals according to FIG. 4 are0.3 ppm for NO and 1 ppm for NO₂.

NO and NO₂ Spectra in Positive and Negative Modes for Complex TransportGases

Laboratory tests were also conducted with more complex, and realistic,transport gases. The transport gas used had the following composition:23.6 ppm of SO₂; 121.2 ppm of H₂; 398 ppm of CO, 8.1% of O₂; 10% of CO₂;with nitrogen as the balance gas. In this part of work the transport gasflow rate was 400 cc/min. As used, the NO₂ flow rate was constant at 5cc/min and concentration of NO₂ in sample mixture was 1500 ppm. NO flowrate varied from 0.62 cc/min to 10 cc/min and concentration of NO insample mixture was 1500 pp. FIG. 5(a,b,c,d) shows the results of theseexperiments.

No peaks in both the positive and negative spectra are evident in thebackground gas mixture shown in FIG. 5 a. This is because all thecomponents of this gas mixture have significantly higher ionizationenergies than the photon energy provided by the UV lamp. Consequently,no positive or negative ions will form directly. After introduction of18 ppm NO₂ the negative ion peak at Vc=−10.5V appeared, and only tracesof the positive ions (see FIG. 5 b) were seen. This condition is asimilar to the case when only clean nitrogen transport gas was passedthrough the DMS (see FIG. 3 b). When NO at 36 ppm was added, FIG. 5 c,simultaneously both positive and negative ion peaks were observed. Thepositive ion peaks have different positions compared to when only thenitrogen transport gas is used, but the negative ion peak has the samecompensation voltage seen for NO₂. One explanation of this result is dueto the presence of oxygen (10%) which can oxidize the NO and produceneutral NO₂ (or NO₃). NO₃ has very high electron affinity and will formnegative ions by direct capture of free electrons, or by electronexchange processes from negative ions of O₂ and CO which may have formedearlier but which have significantly lower electron affinities (EA).Simultaneous introduction of NO (36 ppm) and NO₂ (18 ppm) increases theintensity of the negative ion peak (see FIG. 5 d).

The response of the DMS to different total amounts of NOx is shown inFIG. 6 a. One can see that the peak position for the negative ionsremains the same, and only the peak intensity changes. Concentrationdependence plots for peak area and peak height are shown in FIG. 6 b.The two plots look similar, indicating that the peak form does notchange. The estimated limit of detection for NOx under these conditionsis around 2 ppm.

Effect of Moisture (H₂O) on DMS Negative Ion Spectra

One significant disadvantage of conventional IMS is the suppression ofits response to NOx species as humidity levels increase. The DMSresponse to NOx species at different moisture levels was thereforeclosely studied. FIG. 7, shows results of peak position and peakintensity for the negative ions plotted at different moisture levels.

As is evident in FIG. 7, the moisture does not affect the DMS responseup to about 1000 ppm. Above 1000 ppm the compensation voltage increaseswith increasing humidity, i.e., shifts the peak to a more negative valueaway from the zero axis. This results in an enhanced resolving abilityof the DMS as the moisture level increases. The intensity of peakdecreases, but rather gradually (increasing the moisture level from 1000ppm (relative humidity 4%) to 21000 ppm (relative humidity 85%) changesthe intensity from 6V down to 3.0V) as shown in FIG. 7. Results ofvarying the humidity produce similar peak positions shifts and peakintensities with both the mixture and nitrogen transport gases.

Effect of Propene on Detection of NO/NO₂

Real exhaust typically contains many hydrocarbons. Propene ishistorically used as a representative compound for the hydrocarbonmatrix in exhaust gas. FIG. 8 shows the DMS spectra for a transport gascontaining 100 ppm propene, 10% oxygen in clean nitrogen with moistureat a 70 C dew point.

The resultant spectra shows a propene peak in the positive mode at acompensation voltage of Vc=−26V. The main propene related peak in thenegative mode is at a compensation voltage of Vc=−31V. When a sample of100 ppm NO₂+20 ppm NO is introduced into the DMS, FIG. 9, the positiveion propene peak remains at Vc=−26V while the negative ion peak shiftsto a compensation voltage of Vc=−11 volts. This peak is the same as theone observed for the NO₂ sample. From mass spectrometric analysis thispeak has been determined to be an NO₃ related peak. Once again, thenegative mode DMS spectra shows a characteristic response for the totalNOx sample.

Mass Spectrometric Investigation of DMS Spectral Peaks

Analysis of the DMS spectra with NOx species shows that there are manypossible pathways for formation of these ions in air, especially atambient pressure. Spectra can contain derivative peaks of new specieswhich are formed due to association, dissociation, fragmentation,oxidation and chemical reaction between molecules, fragments, andatmospheric gases. Therefore, validation experiments with directchemical identification have been provided. For this purposemass-spectrometric has been carried out. FIG. 1A 0 shows a schematic ofa preferred micromachined DMS coupled to a mass spectrometer.

A photo-ionization source was attached to the DMS sensor for ionization,and a Teflon base was interfaced to pneumatically attach the DMS to theflange of a TAGA 6000 APCI-tandem mass spectrometer (MS/MS) from Sciex,Inc. (Toronto, Ontario, Canada). The MS/MS was equipped with a computerand API Standard Software, Ver 2.5.1 (PE SCIEX). Analytes (NOx) wereintroduced with transport gas and ionized before introduction into ionfilter region of the DMS. Once through the filter region, the ions wereinjected through a hole in the detector electrode, into the pinhole ofthe interface plate of the MS/MS. Polarity of injected MS ions could bechanged by changing the polarity of the deflector electrode. Thesignificant advantage of this interface is that, under the effect of thedeflector voltage and mass-spectrometer plenum gas, the analyzed ionsare completely isolated from analyte neutrals exhausted by the DMStransport gas. Another advantage of this interface is the possibility tosimultaneously record DMS and MS spectra.

FIG. 11 shows positive ion mass spectra for 20 ppm NO in dry N₂, FIG. 11a, and FIG. 11 b, when 10% of oxygen was added to the dry transport gas(N₂). When instead, NO the NO₂ samples were introduced mass-spectrasimilar to that of FIG. 11 was recorded. One can see that the NOmass-spectra contains two series of ions: related to monomer and clusternitric oxide ions W_(n)NO⁺ and proton bounded water ions W_(k) H⁺. Thelevel of water (W) clustering depends on the moisture level. For dryconditions, with humidity less than 10 ppm; the values of n and k mayvary from 0 to 4. The nitric oxide peaks are M/z=30[NO⁺], 48[(W)NO⁺],66[(W)₂NO⁺], 84[(W)₃NO⁺], 102[(W)₄NO⁺], and proton bound water peaks areM/z=19[W.H⁺], 37 [(W)₂H⁺]; 55[(W)₃H⁺]; 73[(W)₄H₄ ⁺].

When humidity is higher, the level of clustering increases (see FIG. 11c). As a result the light nitric oxide monomer (m/z=30) and cluster(m/z=48) peaks disappear and heavier ions related to NO samples appear:120[(W)₅NO⁺]; 138[(W)₆NO⁺]; 156[(W)₇NO⁺]; 174[(W)₈NO⁺]; 192[(W)₉NO⁺];and 204[(W)₄NO⁺]₂. Likewise, heavier water clusters also appear:91[W₅H⁺]; 109[W₆H⁺]; 127[W₇H⁺]; 145[W₈H⁺]; 163[W₉H⁺];181[W₁₀H⁺];199[W₁₁H⁺].

The nitrogen dioxide and nitric oxide positive ions mass spectra aresignificantly different. In contrast to NO, the nitrogen dioxidepositive ion mass spectra (FIG. 11 b) shows no specific peaks related toNO₂[m/z=46]. The mass-spectrum contains mostly proton bound watercluster ions m/z=37,55,73,91, relatively low intensity peak m/z=30[NO⁺], and m/z=102 [W₄NO⁺]. The last two peaks show that a process ofdissociation of NO₂ molecules to NO+O certainly exists. This is likelykey to considering future oxidizing of NO₂ molecules to NO₃ by a seriesof chemical reaction steps (see, e.g. reactions (1)). The negative ionmass spectra for both samples, provides additional evidence for thismechanism. Mass spectra for both NO and NO₂ samples are similar andcontain peaks m/z=62, 125, 188 with the same peaks intensityrelationship (FIG. 1A 2).

It is likely that these ions are NO₃ ⁻, (HNO₃) NO₃ ⁻, and (HNO₃)₂ NO₃ ⁻which are formed as a result of oxidization (or solvating) of NO and NO₂to NO₃ (or HNO₃). The humidity effect on the negative ion mass-spectrais smaller than on the positive ions. At elevated moisture conditionsthe major peaks are the same as for dry conditions m/z=62, 125, 188, butseveral new cluster peaks at significantly increased mass (Δm=18) alsoappeared (207, 225, 243, 261, etc.). From the mass spectra experimentsit is obvious that water chemistry plays a very significant role in NOxion formation, especially in the positive polarity. With increasedmoisture new cluster ions appear and as a result specific sample ionpeak intensities decrease, due to the increased number of proton boundedwater ions. (This may be an explanation for the suppression ofconventional IMS response.)

According table I the photoionization source could not directly provideionization of water molecules. A mechanism to explain formation ofproton bounded water ions may be described by the following sequence ofgas phase reactions:NO+hv→NO⁺+e⁻NO⁺+W+M→W NO⁺+MW NO⁺+W+M→W₂NO⁺+MW_(n-1)NO⁺+W+M→W_(n)NO⁺+M,  (3)where M is a non-reacting molecule, generally N₂.

When the water cluster around the NO⁺ is sufficiently large, its protonaffinity becomes sufficient for reaction to become exothermic.Theoretical calculations show that this occurs for n=3, i.e. W₃NO⁺ isthe critical size for the switch from NO⁺ chemistry to protonated waterchemistry.W₃NO⁺+W→HNO₂+W₃H⁺  (4)The peak at m/z 102 in the NOx mass-spectra 2 may be explained by thismechanism and may be the W₄NO⁺ ion.Evaluation of Memory Effects on Measurement Precision and Accuracy

In practice of the invention, memory effects were not a significantproblem even though most characterization work was performed at roomtemperature. Furthermore, in real-world application, the gas temperaturewill be at least 100 C, which will reduce or eliminate any memoryeffects should they occur.

Based upon the foregoing, it will be understood that we have shownhighlights of the preferred practice of the present invention. Some ofthe highlights in practice of an illustrative embodiment of theinvention include: NO samples are shown to produce a spectra containingonly positive ion peaks. As expected there are no negative ionsproduced. NO₂ samples produce spectra containing a number of lowintensity peaks in the positive mode (including a peak with the samecompensation voltage as observed with the pure NO sample) and one majorpeak in the negative polarity mode. The NO spectra, even at roomtemperature in the presence of oxygen, is transformed into a spectrumrepresentative of the NO₂ sample. Concentration dependence of NO and NO₂samples have different approximations: NO has linear and NO₂ hasquadratic approximation.

The estimated limit of detection for both species is around 1 ppm. Evenin the presence of interferants, the negative ion spectra is stable andprovides sufficient accuracy for NOx monitoring under real worldconditions.

Mass spectrometric analysis of the positive and negative DMS spectrashow that positive mode DMS peaks are mostly related to derivatives ofNO⁺ and protonated water ions. In the negative mode, the DMS peakcontains ions of NO₃ ⁻ and its derivatives, which are oxidation productsof NO and NO₂. The formation of ions observed in the mass spectra areconsistent with known chemistry but it must be emphasized that thesystems are complex and there may be different channels (pathways) ofions formation which can explain obtained DMS/MS spectra.

It will now be appreciated that the present invention provides directmeasurement of NOx, including NO and NO₂. High spectrometer sensitivityand ability to resolve NOx samples in real-time, even those notseparated in conventional TOF-IMS, has been demonstrated. Furtherapplications include an all-in-one detector that can make measurement ofboth hydrocarbon and NOx content in a gas sample in real-time. Thus suchembodiment enables the realization of fast, miniature, low cost, highsensitivity, high reliability chemical detectors for detection of NOxand even of hydrocarbons in a gas sample, simultaneously.

Furthermore, the present invention enables a compact system for otherdetection needs, such as detection of NO in exhaled breath formonitoring and treatment of medical conditions. The present inventiontherefore has applications to these and other medical treatments as willbe apparent to a person skilled in the art.

Therefore it will now be understood that the present invention disclosesimproved method and apparatus for gas sample analysis. Apparatus of theinvention may feature planar, cylindrical, radial, or other DMStopologies and configurations. A preferred DMS embodiment of theinvention enables a practical, small, fast (<100 msec), non-radioactive,sensitive and selective detector for real time monitoring of NOx.

The examples and embodiments disclosed herein are shown by way ofillustration and not by way of limitation. The scope of these and otherembodiments is limited only as set forth in the following claims.

1. System for chemical analysis of a gas sample, comprising: a flowpath, a soft ionization source, an ion filter including ion filterelectrodes across said flow path, an electronic input, said electronicinput coupled to said ion filter electrodes for generating a high-lowvarying asymmetric displacement field between said electrodes acrosssaid flow path, said asymmetric displacement field imparting transversemotion to said flow of ion species according to mobility characteristicsof said ion species and according to extant field conditions, saidtransverse motion driving unwanted ion species in said flow of ionspecies into said ion filter electrodes for neutralization thereof, saidasymmetric displacement field being compensated, said compensation forselecting at least one ion species out of said flow of ion species andaccommodating travel of said selected ion species in said flow path tosaid ion outlet for detection without said neutralization, said ionfilter distinguishing between ion species of an ionized NOx sample basedon differences in ion mobility in said filter field, and said filterpassing selected said ion species of said ionized NOx sample fordetection based on said compensation, and said detected, passed,selected, ion species being identified based on historical detectiondata of said system and on said extant field conditions 2-20. (canceled)